In optical-fiber light-transmission systems, it is typically important to increase the coupling efficiency between some type of opto-electronic device, such as a light source or detector, and the optical fiber transmission media. One of the most common applications is in communications systems, in which an optical signal, generated by a modulated semiconductor diode, laser diode light source, or a source with a separate modulator, is coupled into a fiber. For long distance transmission applications, the laser diode is usually a single-transverse-mode device, and the fiber only propagates a single mode. At the distant end of the fiber and typically after multiple hops through fiber amplifiers, the optical signal is coupled from the end of the fiber to a detector, where it is converted back to an electrical signal. There are also ancillary applications such as the laser diodes that are used to optically pump the fiber amplifiers. Here, the pump light generated by the laser diode is coupled into a fiber for transmission to the fiber amplifier.
The coupling issues typically surround the interface between the opto-electronic device and the end of a fiber in an opto-electronic module. In the most common implementation, the opto-electronic device, laser diode or detector, is affixed to a substrate, which is installed in the opto-electronic module. The end of a short length of specially sheathed optical fiber, termed the pigtail, is brought into the module, aligned to the opto-electronic device, and secured by reflow soldering, laser welding, or some other technique. Considering the example of modulated sources, such as directly modulated laser diodes or cw laser diodes with modulators, or pumps where the issues of high coupling efficiency are typically most important, there are a number of techniques for maximizing coupling efficiency. Selection between these techniques involves some tradeoffs between three different factors: module-manufacturing complexity, yield, and coupling efficiency.
The simplest coupling approach is optical fiber pigtail cleaving, with butt coupling. The end of the optical fiber is cleaved to a vertical, clean, flat endface. This endface is then secured in the opto-electronic module as close as possible to the light-emitting facet of the opto-electronic device.
Butt coupling has good yield since vertical endface cleaving is easily reproducible. The technique is also the least expensive, again because cleaving is a relatively easy technique. It suffers, however, from poor coupling efficiency. Typically, only about 10% of the light emitted from the opto-electronic source is coupled to be transmitted through the fiber.
In order to increase coupling efficiency over butt coupling, the general technique is to place a convex lens at the end of the fiber pigtail so that more light is collected from the opto-electronic source into the fiber""s core. One of the simplest techniques is to secure a micro-spherical bead to the end of the fiber to function as a lens. Specifically, the end of the fiber is etched to have a concave surface, and then the micro-spherical bead is glued to the fiber""s end. This has advantages in manufacturability and substantially higher coupling efficiencies, approaching 40%.
When still higher coupling efficiencies are required, microlens endfaces are fashioned on the optical fiber pigtail. There are three common techniques for achieving this: pulling, etching, and polishing. Each of these techniques allows for the formation of a fiber endface on the pigtail that closely matches the hyperbolic surface, which is ideal for coupling efficiency, but the techniques offer varying levels of reproducibility.
Fiber pulling involves drawing the fiber in a flame to create a necked-down region and then cleaving the fiber at this narrowed waist, possibly followed with flame fusing. This technique is simple, but the resulting lens has large deviations from a hyperbolic surface.
In the past, many standard laser source""s fiber endfaces where made by etching and then fusing hemispheric lenses. The coupling efficiencies ranged from 50 to 75%. Specifically, the fiber was polished with its jacket in place and then soaked in a heated, buffered hydrofluoric acid solution for 3-4 hours. This was performed until the target end diameter of approximately 13 xcexcm, +/xe2x88x923 xcexcm was produced. An electrode arc was then used to melt and finish the lens. The problem here, however, was concentricity of the lens relative to the fiber""s core, and this process was very dependant upon the fiber jacketing and characteristics of the particular fiber etchant.
Finally, fiber polishing involves using a motor-controlled jig to bring the end of the optical fiber pigtail in contact with a rotating abrasive wheel and polishing the endface to form the intended endface lens. Here, in the past, the lenses have typically been formed with typically steep or obtuse vertex angles.
If high coupling efficiencies are required, lens polishing or etching techniques are among the few options that are available. Fiber pulling techniques yield a lens that deviates too strongly from the optimal hyperbolic endface. Of the two remaining techniques, fiber endface etching is the easiest to perform. The major problem, however, arises in yield. Typically, etch rates for fiber pigtails vary from batch-to-batch. As a result, typically test etching must be performed to determine the optimal solution concentrations and etch times. Further, even when these factors are optimized, the etched lens may not be concentric with respect to the optical fiber pigtail""s core, which substantially impairs coupling efficiency to render the nonconcentric fibers useless.
This leaves endface polishing as the remaining choice. There are a number of problems, however, with existing techniques. Typical fiber polishing techniques yield a very conical endface surface which deviates from the optimal hyperbolic surface. Further, research has shown that the common obtuse vertex angles are sub-optimal for coupling efficiency.
The present invention involves a number of innovations in optical fiber endface polishing that substantially increase coupling efficiency while simultaneously improving yield. Specifically, the endface of the fiber is polished, preferably to form a frustum-like or pointed endface. However, after polishing, it is fused, preferably in an arc fuser to form a smooth, rounded endface. Research has additionally shown that the vertex angles used in most polishing techniques are too large. In this way, older fiber drawing techniques offered certain advantages. As a result, according to the invention, the endface of the fiber is polished to have an acute vertex angle, resulting in substantial improvements in coupling efficiency.
In general, according to one aspect, the invention features an endface of an optical fiber. The endface has a frustum region, which is produced by polishing the endface. In order to produce the optimal rounded and smooth tip, however, the polished fiber is then fused.
In specific embodiments, the polished frustum region is frusto-conical, or near conical. Such circular symmetry is desirable where the opto-electronic source has high far field symmetry as in the case with most modulated opto-electronic sources.
Preferably, to increase coupling efficiency, the vertex angle of the polished frustum region is acute, preferably between 16xc2x0 and 20xc2x0. Research has shown this to be the optimum range. Further, match the desired hyperbolic surface, an intermediate frustum region is preferably polished into the fiber.
In general, according to another aspect, the invention concerns an opto-electronic module. This module contains an opto-electronic device, such as a laser diode or laser diode/modulator, or even an opto-electronic detector. An optical fiber in the form of a pigtail transmits the optical signal either from or to the opto-electronic device. This pigtail, according to the invention, has an endface that has a polished frustum region and a fused tip.
In general, according to another aspect, the invention also features a method for manufacturing a lens at the end of an optical fiber. The method comprises polishing the end while rotating the optical fiber around its axis. Then, according to the invention, the tip of the fiber is fused.
In specific embodiments, the concentricity of the fiber is checked during the polishing step. Research has shown that the concentricity must be controlled in order to maximize the coupling efficiency. The lack of this control was one of the primary drawbacks associated with the fiber endface etching techniques.
In the preferred embodiment, an endface with two or more discrete vertex angles is produced by polishing as the endface is placed in contact with a polishing wheel.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.